In internal combustion engines for automobiles, the connecting rod is used to connect the piston to the crankshaft. The connecting rods are usually made of metal, such as steel, although also other materials can be used, for example, to achieve a desired lightness. A connecting rod typically has a small end with a small bore, and a big end with a big bore. The small end attaches to the piston pin or similar, and the big end typically connects to the crankshaft.
FIG. 1 illustrates a typical design of a connecting rod. The connecting rod 1000 comprises what will hereinafter be referred to as a rod portion or rod 1001 (comprising the stem portion 1004 and the small end 1003 in which the small bore 1005 is formed, as well as part of the big end in which the big bore 1006 is formed), and a cap 1002 which, together with the big end of the rod 1001, defines the big bore 1006. The cap 1002 is attached to the rod by screws 1007 which are screwed into corresponding bores. The joint 1008 between rod 1001 and cap 1002 is often hardly visible on the finished rod.
The connecting rod is obtained by machining a blank in the form of a single metal piece so as to produce a rod blank comprising both the small end with the small bore and the big end with the big bore. This rod blank is subsequently split into the rod 1001 and the cap 1002. This operation is normally referred to as “cracking” of the rod, an operation which is normally performed by introducing an object, such as two expander mandrel parts, into the big bore 1006, and separating these two mandrel parts using, for example, a wedge element. Before performing the actual cracking, the big end has been “notched” by, for example, laser or other suitable means, so as to establish notches that define the plane where the big end will split during separation of the two expander mandrel parts.
Due to the fact that the cap 1002 and rod 1001 are separated from each other by actually “cracking” the metal (instead of, for example, by cutting or other means, or instead of producing rod and cap from two separate blanks), the surfaces where rod and cap are connected to each other fit very well together, once the rod and cap have been reassembled to form the connecting rod.
Cracking rod blanks is conventional in the art of manufacture of connecting rods.
For example, DE-19841027-C1 describes a machine used for cracking rods. A laser is used to produce the notches that define the plane of cracking. Cracking is performed by using what appears to be two expander mandrel halves, which expand due to the movement of a wedge element. The machine further comprises means for screwing the cap onto the rod.
U.S. Pat. No. 6,457,621-B1 teaches a device for separating rod and cap of a connecting rod, by means of cracking the big end using two expander mandrel halves and a cleaving wedge. The device comprises a fixed device half and a movable device half. U.S. Pat. No. 6,457,621-B1 describes extensively how the rod blank can be fixed in its position during the operation.
Also DE-9320463-U1, EP-568119-A1 and EP-467198-A1 describe different arrangements for cracking connecting rods.
U.S. Pat. No. 6,671,955-B1 describes a method for treating a connecting rod after cracking, by applying a vibration treatment. The crack surfaces are in contact during the vibration treatment.